This invention relates to an electric conductive roller which is used for electrophotographic apparatuses such as copying machine, printer, facsimile and the like.
In various electrophotographic apparatuses, there have hitherto been used an electric conductive roller which is charged or discharged by applying a voltage to a roller shaft to bring the surface of the roller into contact with a charged material.
That is, in Japanese Laid-Open Patent Publication No. 5-331307, there is disclosed an electric conductive roller obtained by mixing carbon black as an electric conductive substance with an ethylene-propylene-diene copolymer rubber (EPDM) and subjecting the resulting blend to foam molding.
Further, in Japanese Patent Publication No. 5-40772, there is disclosed an electric conductive polyurethane foam obtained by mixing a quaternary ammonium salt with a polyurethane foam and subjecting the blend to foam casting.
It is necessary for the above electric conductive roller, wherein carbon black is mixed in the ethylene-propylene-diene copolymer rubber, to mix a large amount of carbon black so as to obtain a desired electric resistance value. Therefore, the electric resistance of the roller varies as a function of a change in applied voltage. Resistance which has such a dependence on the applied voltage requires a precision applied voltage control apparatus so as to obtain a requisite transfer current when the electric conductive roller is used for the electrophotographic apparatus, thereby causing a problem of an increase in cost.
On the other hand, in the electric conductive roller obtained by mixing the quaternary ammonium salt with polyurethane and foaming the blend, the electric resistance depends upon the amount of the quaternary ammonium salt to be mixed. Since the polyurethane itself has semiconducting properties, its resistance does not significantly vary depending on the applied voltage. However, when a hydrophilic quaternary ammonium salt is additionally mixed in a hydrophilic polymer, the changes in electric resistance due to a change in environment (e.g. temperature, humidity, etc.) is large.
Further, it has also been known to set the electric resistance at a desired value by only using a low-resistance rubber without mixing carbon black, quaternary ammonium salt, etc. there with. The electric conductive roller thus obtained has a problem in that it's change in resistance due to environmental change is large, but the change in electric resistance is not as large as that in case of the combination of the polyurethane with quaternary ammonium salt.
Therefore, it has hitherto been desired to develop an electric conductive roller which is stable to a change in applied voltage and environment.